1. Field of the Invention
The invention relates to photon collimators, and more particularly collimators suitable for medical imaging apparatus that are fabricated with electric discharge machining (EDM) techniques or other thermal ablation cutting techniques, such as laser cutting.
2. Description of the Prior Art
Photon collimators are utilized in medical imaging and therapy apparatus, such as gamma cameras, to allow passage of photons that have trajectory paths aligned with apertures that are defined by the collimator structure. Photons having non-aligned trajectory paths are blocked by the collimator structure. Known collimators are shown in FIGS. 1 and 2, and are often fabricated from relatively dense material-commonly lead alloys. FIG. 1 depicts a known type of cast lead alloy collimator 20, having a matrix-like array of cast-in-place aperture through holes 22 that are aligned along respective length and width axes 24, 26. The apertures 22 are often formed by casting molten lead around a matrix grid of mold pins (not shown).
FIG. 2 depicts another known type of fabricated lead alloy collimator 30 having a matrix-like array of aperture through holes 32 that is formed from a repetitive pattern of opposing lead foil strips 34, 36 that are bonded together with a layer of glue 38. Each opposed face of the foil strips 34, 36 is calendered (i.e., compressed or squeezed) with a series of half-polygonal (e.g., semi-circular or half-hexagonal) impressions that when joined together in opposed fashion along the glue layer 38 form each individual aperture 32. Adjoining pairs of lead strips 34, 36 are in turn bonded with glue layers 38 to form a unitized, fabricated collimator structure 30.
Some jurisdictions are discouraging use of lead components in general, including medical equipment. Hence, there is a perceived need to replace lead alloys in medical and other equipment collimators with substitute dense alloy materials. Tungsten (W) and Molybdenum (Mo) alloys are being considered as lead substitutes in collimators, but their hardness and relatively high melting temperatures make them more difficult to fabricate for collimators. Some low energy-level collimators have been fabricated from molybdenum foil, but the foil thickness is too thin for the photon energy levels and density normally required for human medical imaging apparatus. Some attempts have been made to fabricate smaller collimators not suitable for human medical imaging by laser sintering molybdenum and tungsten powders. However, laser sintering complex grid patterns of the size necessary for medical imaging photon collimators is relatively time consuming and expensive.
Tungsten and molybdenum alloys are not as easily cast as lead to meet high precision tolerances required for medical imaging collimators—often requiring less than 1/20  degree variation between adjoining apertures. Due to material hardness properties, tungsten alloys are not readily calendered in precision foil strips of sufficient thickness for medical imaging collimators, or readily mechanically machined (e.g., by drilling or milling). It is difficult to maintain inter-aperture size, shape and spacing variation within acceptable tolerances by mechanical machining techniques. While some relatively small collimators for less than human size imaging apparatus have been fabricated by EDM cutting individual apertures, a typical gamma camera collimator for human patients requires fabrication of thousands of aperture holes in a precision matrix-like grid. Mechanically machining or thermal ablation cutting such a large quantity of individual apertures is time consuming and costly.
Thus, a need exists in the art for a photon collimator that can be fabricated with inter-aperture size, shape and/or spacing variation tolerances required for human-sized medical imaging equipment.
Another need exists in the art for a non-lead alloy photon collimator that can be fabricated from tungsten, molybdenum or other photon-attenuating alloys with known, cost-effective machining techniques and equipment.
An additional need exists in the art for a photon collimator that can be fabricated with fewer forming operations otherwise required to fabricate a matrix-like grid of individual aperture holes on a one-by-one basis.